Overlay material for plain bearing comprising filled fluorothermoplastic material

ABSTRACT

A sliding bearing material contains a matrix material and an antifriction material composed of at least one fluorinated plastic material and fillers. The antifriction material contains as fillers 5 to 48% by volume boron nitride and 2 to 45% by volume of at least one metal compound with a stratified structure, the proportion of fluorinated thermoplastic material amounting to at least 50 to 85% by volume. The fluorinated thermoplastic material is a PTFE or PTFE with additives; M o  S 2 , tungsten, sulphide, titanium sulphide or titanium iodide may be used as metal compounds. The matrix material may be a sintered bronze into which the antifriction material is incorporated or a thermoplastic material in which the antifriction material is finely distributed. The proportion of plastic matrix material in the whole sliding bearing material lies at 60 to 95% by volume.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a plain bearing material comprising a matrixmaterial and a sliding material of at least one fluorothermoplastic andfillers. The invention also relates to the use of such plain bearingmaterials.

2. Description of Related Art

Bearing materials with plastics-based overlays are known assingle-layer, two-layer or three-layer composite materials: solidplastics bearings, bearings with an outer metallic backing and directlyapplied or adhered plastics, other such with inner wire meshes, as wellas three-layer bearings of backing metal, a sintered porous metal layerand a covering layer formed in the pores. All these bearings aregenerally used in areas in which the use of lubricants is impossible orundesirable. For this reason, they must provide these lubricantsthemselves when in operation.

Multilayer materials differ from solid plastics materials, for exampleby a negligible tendency towards cold flow under load, by substantiallybetter heat conductivity and, in connection therewith, by markedlyhigher possible pv values. However, solid plastics materials may also beadvantageous in certain cases, e.g. for reasons of cost.

Among three-layer materials, it is possible to distinguish furtherbetween those with overlays based on fluorothermoplastics, such aspolytetrafluoroethylene (PTFE), perfluoroalkoxyalkane (PFA),perfluoroethylene-propylene (FEP) etc., and those with overlays based onother plastics, such as polyetheretherketone (PEEK) for example. Thelatter two groups differ in their manner of operation: while, in thecase of PTFE-based materials, the bronze intermediate layer is the"active" load-bearing component of the overlay and acts like a filler,the other plastics materials use it only as an anchoring means. If thereis sufficient affinity to the metal backing, they permit the productionof true two-layer materials, but may also be applied with the aid of anadhesive. On the active overlay itself the thermoset or thermoplasticsmaterial then assumes the supporting role of the bronze. Bearingmaterials of filled fluorothermoplastic films adhered to metal or othersuch materials with wire meshes incorporated in plastics are also known,which may likewise be adhered to a metal backing.

For universal applicability and ease of production, the mostadvantageous materials are three-layer materials based onfluorothermoplastics such as PTFE, which also exhibit the highestperformance and temperature-resistance. In the production process,homogeneous PTFE/filler pastes are produced by means of a plasticsdispersion and the final composite material is produced by a concludingstep comprising sintering of the PTFE subsequent to rolling thereof ontothe backing material.

The most commonly used fillers for such materials are lead andmolybdenum disulphide, these materials providing virtually equalperformance levels. These fillers may also be used in the presence oflubricants.

In many cases, it would be desirable to be able to solve constructionalproblems by using maintenance-free, space-saving plain bearings withPTFE overlays. However, the upper load limit, which, at a pv value of 2MPa m/s, lies within the average loading and speed range (0.5-100 MPAand 0.02-2 m/s), may restrict use of these plain bearings.

It is known from DE 41 06 001 A1 that plain bearing materials withbetter performance levels may be also produced by using PbO as thefiller, but there is a growing stigma attached to the use of materials,such as lead, which are potentially damaging to health. Furthermore,such materials, which are of optimum suitability for lubricant-free use,are unsuited for example to use as guide bushings for shock absorberpiston rods, because under these conditions they do not exhibit thenecessary wear and cavitation resistance or else their coefficients offriction are unsatisfactorily high.

Bearing materials with overlays consisting only of PTFE and molybdenumdisulphide have long been known and are currently some of the mostfrequently used lubricant-free bearing materials. Boron nitride, whichis known to be a solid lubricant, is also repeatedly named as a possiblefiller for PTFE despite the fact that the lubricant properties then onlybecome effective at temperatures of over 800° C.

Thus, both molybdenum sulphide and boron nitride are already mentionedin DE-PS 11 32 710 as Examples in a list of solid lubricants. Thesimultaneous use of molybdenum disulphide and boron nitride in PTFE isnot mentioned, however.

SUMMARY OF THE INVENTION

The problem on which the invention is based is that of providing afiller combination which extends the above-mentioned advantages ofthermoplastics-containing plastics bearing materials without recourse tolead or lead compounds.

This problem is solved in that the sliding material contains as fillersfrom 5-48 vol. % boron nitride and from 2-45 vol. % of at least onemetal compound with a laminar structure, the proportion offluorothermoplastics amounting to from 50-85 vol. %. The volumepercentages are based on the sliding material without the matrixmaterial.

It has been shown that, by using a filler combination of boron nitride,wherein boron nitride in its hexagonal modification is preferred, and atleast one metal compound with a laminar structure, the properties ofplastics bearing materials may be markedly improved over bearingmaterials which contain only one of these fillers. These propertiesinclude, in particular, loadability, wear resistance and cavitationresistance.

The laminar structure of the metal compounds should be understood tomean anisotropy of the crystal lattice such that displacement of certainlattice planes with respect to each other is made easier by weakinteractions of the constituent parts of the atom upon the applicationof external force. Possible metal compounds are, for example, molybdenumdisulphide and/or tungsten sulphide and/or titanium sulphide and/ortitanium iodide.

The fluorothermoplastics are preferably PTFE or PTFE with the additionof one or more fluorothermoplastics selected fromethylene-tetrafluoroethylene (ETFE), polychlorotrifluoroethylene(PCTFE), ethylene-chlorotrifluoroethylene (ECTFE), polyvinylidenefluoride (PVDF) or FEP.

The advantageous properties are particularly obvious when the matrixmaterial is a bronze framework into which the sliding material isinserted, or when the matrix material is a thermoplastic into which thesliding material is mixed in fine dispersion, the proportion of plasticsmatrix material based on the total plain bearing material amounting tofrom 60-95 vol. %, preferably from 70-90 vol. %.

The plastics matrix material preferably comprises polyphenylenesulfide(PPS), polyamide (PA), polyvinylidene fluoride (PVDF), polysulfone(PSU), polyethersulfone (PES), polyetherimide (PEI), PEEK,polyamideimide (PAI) or polyimide (PI).

In PTFE/sintered bronze-based plain bearing materials the performancelevel is so improved that, under lubricant-free conditions, pv valuesmay be achieved in the average load and speed range of up to 5 MPa m/s.At the same time, these materials are distinguished by improvedcavitation resistance and by coefficients of friction with oillubrication which are comparable to the prior art. Even withfluorothermoplastics-containing materials with a different plasticsmatrix, such as PPS, PA, PVDF, PSU, PES, PEI, PEEK, PAI or PI, thefiller combination according to the invention may provide a markedimprovement in wear resistance in comparison with variants filled withsingle components.

The proportion of boron nitride is preferably between 6.25 and 32 vol. %and that of the metal compound is preferably between 5 and 30 vol. %.The particle size of these fillers is preferably below 40 μm, especiallybelow 20 μm.

Proportions other than those according to the invention do not provideany substantial improvements over the combinations of PTFE and boronnitride or PTFE and MoS₂. However, it is possible, starting with theabove-mentioned compositions, to replace up to 40 vol. % of the boronnitride/metal sulphide filler combination, but preferably not more than20 vol. %, with other components such as thermoset or high temperaturethermoplastics materials, such as polyimides or polyamide imides forexample, other solid lubricants, e.g. graphite, pigments such as cokefor example, and fibrous materials such as short graphite fibres oraramid fibres or hard substances such as boron carbide or siliconnitride for example.

The sliding material according to the invention may be used inconjunction with a matrix material of thermoplastics as a solid plasticssliding element. However, the plain bearing material may also find useas an overlay for a multilayer material, it being possible to apply theoverlay directly to a metal backing. The latter multilayer materialswith an overlay comprising a sintered bronze matrix are so constructedthat a 0.05-0.5 mm thick layer of bronze is sintered onto the backingmetal, such as steel or a copper or aluminium alloy for example, in sucha way that it comprises a pore volume of from 20-45% and the compositionof the bronze itself contains from 5-15% tin and optionally up to 15%lead. The sliding material mixture is then applied by rolling onto theporous substrate in such a way that the pores are completely filled and,depending on the use to which it is going to be put, a 0-50 μM thickcovering layer arises. The material is then subjected to heat treatmentin an oven, during which the PTFE obtained is sintered for three minutesat 380° C. in order, in a concluding rolling step, to produce the finalcomposite and the necessary final dimensions.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments are described in more detail below with the aid ofthe Figures and Tables. In the Figures:

FIG. 1 shows the rate of wear in dependence on the boron nitride contentof the filler, and

FIGS. 2-5 are graphic representations of the test results assembled inthe Tables.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The sliding material mixture may be produced, as is described in Example1 below, by means of a PTFE dispersion, to which the fillers are addedin such a way that they are in homogeneous dispersion when coagulationis subsequently effected. A pasty composition then arises, whichcomprises the properties necessary for the subsequent coating process.

EXAMPLE 1

12 l of water, 25 g sodium lauryl sulphate, 3 kg boron nitride, 15.9 kgmolybdenum sulphide and 34 kg of a 35% PTFE dispersion are stirredvigorously for 20 minutes. 100 g of a 20% aluminium nitrate solution arethen added. After the completion of coagulation, 1 l toluene is stirredinto the mixture and the escaped fluid is removed.

The other Examples cited in Tables 1 to 4 from the group comprisingthree-layer systems with a PTFE/bronze matrix overlay may all beproduced in this way. Below, therefore, only the compositions of theplastics mixtures are mentioned.

The materials produced in the manner described are markedly superior inthe compositions according to the invention to the standard materialsbased on PTFE/MoS₂ or PTFE/Pb with respect to both the coefficient offriction and wear resistance.

In order to examine properties such as wear resistance and thecoefficient of friction, the compositions of PTFE, boron nitride andmolybdenum disulphide were varied over a wide range and samples of theabove-described three-layer materials were produced comprising 1.25 mmsteel, 0.23 mm bronze and 0.02 mm of a plastics covering layer. Therates of wear of these samples at a peripheral speed of 0.52 m/s andunder a load of 17.5 MPa were measured using 0.78 cm² specimens and apin/roller tribometer and compared with a standard material. Thestandard material used was a composite material of the above-describedtype with a plastics layer consisting of 80 vol. % PTFE and 20 vol. %MoS₂.

FIG. 1 shows the rate of wear in dependence on the boron nitride contentof the filler, the total filler being kept constant at 30 vol. %. It maybe clearly seen that, within the hatched range according to theinvention, there is a pronounced point of minimum wear which reveals thecompositions at their best to be approximately four times as good asthose which contain only molybdenum sulphide for example.

To clarify the improvements achievable according to the invention, thematerial compositions listed in Table 1 together with the coefficientsof friction and rates of wear obtained by the pin/roller test wereadditionally tested. FIG. 2 graphically compares the results, which showthat the filler combination performs better in every instance than justone of the two components.

It may be seen from the results obtained with example compositions 7-11and listed in Table 2 and FIG. 3 that the materials according to theinvention may also be combined with other components without thepositive properties being lost. It may be seen that further improvementsmay be achieved by such additions.

Furthermore, the efficacy of tungsten sulphide was tested. Thecorresponding result from the pin/roller test stand is shown in Table 3.It is clear that, to achieve the effect according to the invention,other materials which are structurally similar to molybdenum disulphidemay also be suitable.

Bushings 22 mm in diameter were produced from example compositions nos.1 and 3 and their loadability limit was tested in a rotating test run.The highest possible load at which a running distance of 13.5 km wasachieved at a speed of 0.075 m/s was defined as the limit load. Thefailure criterium was a sharp rise in temperature, which proved, onsubsequent examination, to be synonymous with an average wear depth of90 μm. The result of sample 3 corresponds, when differently evaluated,to a pv value of 4.5 MPa m/s. In FIG. 4 the results are compared withthe prior art.

Table 4 makes it plain that the performance of the materials accordingto the invention when used in shock absorbers is equal to the prior art.Both the cavitation resistance and the coefficient of friction are ofcomparable level. The Table is based on a shock absorber testing programwith extreme levels of cavitation stress. The failure criterium in thedetermination of service life was here complete and partial slidingsurface detachment. The coefficients of friction were determined usingbushings with the above-mentioned dimensions operating against shockabsorber piston rods under a 1000 N load and at a sliding speed of 20mm/s. Drip feed lubrication was provided.

Another possible way of advantageously realising the invention comprisesincorporating the sliding material mixtures with PTFE according to theinvention into a thermoplastic matrix of another polymer such as, forexample, PPS, PA, PVDF, PES, PSU, PEEK, PI, PA or PEI and thenprocessing this combination in any desired way to form a slidingelement, e.g. applying it to a metal backing with or without a bronzeintermediate layer or producing solid plastics parts. The thermoplasticscontent may vary between 60 and 95 vol. %, preferably between 70 and 90vol. %.

The fluorothermoplastics/boron nitride/molybdenum disulphide mixtureaccording to the invention, in the form of a dry blend, is dispersedover a steel/bronze substrate and fused and rolled thereon. However, itis also possible to produce the mixture by melt compounding. Theinfluence on the tribological properties of a PES compound serves as theexample here, but many other thermoplastics may also be used as thematrix.

To clarify the effect according to the invention, FIG. 5 shows thecoefficients of friction and the wear values of PES compounds with PTFE,PTFE/MoS², PTFE/BN and PTFE/BN/MoS₂. The precise compositions andmeasured values are give in Table 5. The best values are achieved byExample 14.

                  TABLE 1                                                         ______________________________________                                                                  Wear    Coefficient                                 Ex. No. Composition in vol. %                                                                           [μm/h]                                                                             of friction                                 ______________________________________                                        PRIOR ART                                                                             PTFE 80, MoS.sub.2 20                                                                           120     0.20                                        1       PTFE 55, BN 12.8, MoS.sub.2 32.2                                                                36      0.23                                        2       PTFE 60, BN 20, MoS.sub.2 20                                                                    35      0.23                                        3       PTFE 65, BN 12.5, MoS.sub.2 22.5                                                                32      0.21                                        4       PTFE 70, BN 15, MoS.sub.2 15                                                                    39      0.23                                        5       PTFE 75, BN 7.2, MoS.sub.2 17.8                                                                 40      0.20                                        ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                                                   Wear    Coefficient                                Ex. No. Composition in vol. %                                                                            [μm/h]                                                                             of friction                                ______________________________________                                        4 (for  PTFE 70, BN 15, MoS.sub.2 15                                                                     39      0.23                                       comparison)                                                                   6       PTFE 70, BN 13, MoS.sub.2 13, Coke 4                                                             26      0.18                                       7       PTFE 70, BN 13, MoS.sub.2 13,                                                                    35      0.24                                               Si.sub.3 N.sub.4 4                                                    8       PTFE 70, BN 13, MoS.sub.2 13,                                                                    28      0.21                                               C fibres 4                                                            9       PTFE 70, BN 13, MoS.sub.2 13, PI 4                                                               25      0.18                                       ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                                                 Wear     Coefficient                                 Ex. No. Composition in vol. %                                                                          [μm/h]                                                                              of friction                                 ______________________________________                                        4 (for  PTFE 70, BN 15, MoS.sub.2 15                                                                   39       0.23                                        comparison)                                                                   10      PTFE 70, BN 15, WS.sub.2 15                                                                    36       0.22                                        ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                                                  Wear    Coefficient                                 Ex. No.                                                                              Composition in vol. %                                                                            [μm/h]                                                                             of friction                                 ______________________________________                                        prior art                                                                            PTFE 80, MoS.sub.2 20                                                                            37      0.018                                       3      PTFE 65, BN 12.5, MoS.sub.2 22.5                                                                 40      0.020                                       5      PTFE 75, BN 7.2, MoS.sub.2 17.8                                                                  33      0.019                                       ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                                                  Wear    Coefficient                                 Ex. No.                                                                              Composition in vol. %                                                                            [μm/h]                                                                             of friction                                 ______________________________________                                        11     PES 80, PTFE 20    35      0.18                                        12     PES 80, PTFE 14, MoS.sub.2 6                                                                     31      0.18                                        13     PES 80, PTFE 14, BN 6                                                                            88      0.17                                        14     PES 80, PTFE 14, BN 3, MoS.sub.2 3                                                               20      0.15                                        ______________________________________                                    

Obviously, numerous modifications and variations of the presentinvention are possible in the light of the above teachings. It istherefore to be understood that within the scope of the appended claims,the invention may be practiced otherwise than as specifically describedherein.

What is claimed is:
 1. A plain bearing material comprising a matrixmaterial and a composite sliding material, said sliding materialincluding a proportion of at least one fluorothermoplastic materialranging in volume percent from 50 to 85% and a proportion of fillerscomprising 5 to 48 vol. % boron nitride and said fillers furthercomprising between 2 to 45 vol. % of at least one metal compound havinga laminar structure and wherein the vol. % of said fillers is expressedin relation to the total volume of the sliding material.
 2. The plainbearing material of claim 1 wherein said fluorothermoplastic materialcomprises polytetrafluoroethylene (PTFE).
 3. The plain bearing materialof claim 1 wherein said fluorothermoplastic material comprises PTFE andat least one fluorothermoplastics selected from the group consisting of:ethylene-tetrafluoroethylene (ETFE).
 4. The plain bearing material ofclaim 1 wherein said at least one compound is selected from the groupconsisting essentially of: molybdenum sulfide, tungsten sulfide,titanium sulfide, and titanium iodide.
 5. The plain bearing material ofclaim 1 wherein said boron nitride is present in hexagonal modification.6. The plain bearing material of claim 1 wherein said proportion ofboron nitride ranges from 6.25 to 32 vol. % and the proportion of saidmetal compound ranges from 5 to 30 vol. %.
 7. The plain bearing materialof claim 1 wherein said fillers have a particle size of less than 40 μm.8. The plain bearing material of claim 1 wherein said fillers have aparticle size less than 20 μm.
 9. The plain bearing material of claim 1wherein said sliding material includes up to 40 vol. % additives otherthan said boron nitride/metal compound filler combination.
 10. The plainbearing material of claim 9 wherein said additives include materialsselected from the group consisting essentially of: hard substancesincluding silicon nitride and boron carbide, pigments including coke,fibrous materials including short graphite fibers and aramid fibers,solid lubricants including graphite, and high temperature thermoplasticsincluding polyamideimide (PAI) and polyimide (PI).
 11. The plain bearingmaterial of claim 1 wherein said matrix material comprises a porousbronze framework into which said sliding material is introduced.
 12. Theplain bearing material of claim 1 wherein said matrix material comprisesa thermoplastic in which said sliding material is dispersed.
 13. Theplain bearing of claim 12 wherein said thermoplastic matrix material ispresent in the range of about 60 to 95 vol. % based on the total volumeof plain bearing material.
 14. The plain bearing material of claim 12wherein said thermoplastic matrix material is selected from the group ofmaterials consisting essentially of: PPS, PA, PVDF, PSU, PES, PEI, PEEK,PAI, and PI.
 15. A solid plastics sliding element comprising a matrixmaterial and a composite sliding material, said sliding materialincluding a proportion of at least one fluorothermoplastic materialranging in volume percent from 50 to 85% and a proportion of fillerscomprising 5 to 48 vol. % boron nitride and said fillers furthercomprising between 2 to 45 vol. % of at least one metal compound havinga laminar structure and wherein the vol. % of said fillers is expressedin relation to the total volume of the sliding material.
 16. A plainbearing comprising a metal backing and an overlay applied to saidbacking, said overlay comprising a matrix material and a compositesliding material, said sliding material including a proportion of atleast one fluorothermoplastic material ranging in volume percent from 50to 85% and a proportion of fillers comprising 5 to 48 vol. % boronnitride and said fillers further comprising between 2 to 45 vol. % of atleast one metal compound having a laminar structure and wherein the vol.% of said fillers is expressed in relation to the total volume of thesliding material.
 17. A plain bearing material comprising a matrixmaterial and a composite sliding material, said sliding materialincluding a proportion of at least one fluorothermoplastic materialranging in volume percent from 50 to 85% and a proportion of fillerscomprising 5 to 48 vol. % boron nitride and said fillers furthercomprising between 2 to 45 vol. % of at least one metal compound havinga laminar structure selected from the group consisting of MoS₂, tungstensulfide, titanium sulfide, and titanium iodide and wherein the vol. % ofsaid fillers is expressed in relation to the total volume of the slidingmaterial.